1. Field of the Invention
The present invention relates to a mobile communication system, and, particularly, to a circuit for measuring voltage standing wave ratios (VSWR) of transmitting and receiving antennas in a base station.
2. Discussion of Related Art
VSWR of a transmitting/receiving antenna of a base station indicates that the ratio of the maximum value and minimum value of a standing wave is 1 in matching state, the standing wave being generated by interference of an incident wave and reflected wave when connecting a load impedance in a transmission line. If the standing wave does not equal to a characteristic impedance in a limited transmission line, some of the incident waves are reflected thereby to generate the reflected waves. The VSWR influences the performance of the transmitting/receiving antenna of the base station. Therefore, it is necessary to accurately measure the VSWR and implement the matching state.
FIGS. 1 and 2 are block diagrams respectively showing transmitting and receiving antenna VSWR measuring circuits of the base station. The forward path indicates a path through which signals are transmitted from the base station to the antenna. The reverse path indicates a path through which signals are transmitted from the antenna to the base station.
As shown in FIG. 1, the transmitting antenna VSWR measuring circuit includes: variable attenuators 12 and 15 coupled to a directional coupler 2 located next to the transmitting antenna 3 for attenuating radio frequency signals; diode detectors 13 and 16 respectively coupled to the variable attenuators 12 and 15 for converting the attenuated radio frequency signals into direct voltage; and voltage comparators 14 and 17 respectively connecting the diode detectors 13 and 16 to a measuring controller (not shown in the drawings) for comparing the converted direct voltage from the diode detectors 13 and 16 with threshold voltage and outputting the results of comparison to the measuring controller.
The following description relates to transmitting power and reflection power measuring procedures of the transmitting antenna VSWR measuring circuit of the base station in the conventional mobile communication system.
For the transmitting antenna transmitting power measuring procedure, the radio frequency signal which is received by the transmitting antenna 3 and passes the forward path or the directional coupler 2 is converted into the direct voltage by the diode detector 13 before being input to the voltage comparator 14. The measuring controller controls analog working voltage of the variable attenuator 12 and detects the analog working voltage of the variable attenuator 12 when the output of the voltage comparator 14 is "HIGH" thereby to measure the transmitting power.
For the transmitting antenna reflection power measuring procedure, the radio frequency signal which is received by the transmitting antenna 3 and passes the reverse path of the directional coupler 2 is converted into the direct voltage by the diode detector 16 before being input to the voltage comparator 17. The measuring controller controls analog working voltage of the variable attenuator 15 and detects the analog working voltage of the variable attenuator 15 when the output of the voltage comparator 17 is "HIGH" thereby to measure the reflection power. At this time, the VSWR of the transmitting antenna can be obtained using the measured transmitting power and reflection power.
As shown in FIG. 2, the receiving antenna VSWR measuring circuit 31 of the base station in the conventional mobile communication system includes: an amplifier 32 coupled to a directional coupler 22 located next to the receiving antenna 23 for amplifying a radio frequency signal, a local oscillator 33 coupled to the amplifier 32 for generating a signal of a predetermined frequency band and applying it to the amplifier 32; a frequency mixer 34 for down-converting the radio frequency signal from the directional coupler 22 into an intermediate frequency (IF) signal; a local oscillator 36 coupled to the frequency mixer 34 for generating a signal of a predetermined frequency band and applying it to the frequency mixer 34; and a receiving signal strength indicator 35 connecting the frequency mixer 34 to the measuring controller (not shown in the drawings) for converting the IF signal from the frequency mixer 34 into the direct voltage and outputting the result of conversion to the measuring controller.
The following description relates to receiving power and reflection power measuring procedures of the receiving antenna VSWR measuring circuit of the base station in the conventional mobile communication system.
For the receiving antenna receiving power measuring procedure, the radio frequency signal which is received by the receiving antenna 23 and passes the forward path of the directional coupler 22 is down-converted into the IF by the frequency mixer 14 before being input to the receiving signal strength indicator 35. The receiving signal strength indicator 35 outputs the strength of the receiving signal converted into analog voltage to the measuring controller which measures the receiving power from the analog voltage.
For the receiving antenna reflection power measuring procedure, the signal which is sent from the local oscillator 33 via the directional coupler 22 is reflected by the receiving antenna 23 and input to the frequency mixer 34 after passing through the reverse path of the directional coupler 22. The reflected signal is down-converted into the IF signal by the frequency mixer 34 and then converted into the analog voltage by the receiving signal strength indicator 35, before being sent to the measuring controller. The measuring controller measures the reflection power from the analog voltage applied by the receiving signal strength indicator 35. Therefore, the VSWR of the receiving antenna can be obtained using the measured receiving power and reflection power.
According to the transmitting and receiving antenna VSWR measuring circuits of the base station in the conventional mobile communication system, however, the transmitting power and receiving power of each frequency band cannot be measured in a multi-frequency allocation system, and the transmitting and receiving antenna VSWRs cannot be accurately measured when a noise signal exists in an adjacent frequency band.